1. Field of the Invention
This invention relates generally to electrical control and supply systems and, more particularly, to intrinsically safe power sources for use in explosive or potentially explosive atmospheric conditions such as those found in the underground coal mining industry.
2. Description of the Prior Art
Numerous national and international standards such as UL913 define requirements for intrinsic safety. These standards require that intrinsic safety be maintained with two component failures and any subsequent failures resulting from the first two component failures. In the underground coal mining industry, tests for intrinsic safety are specifically defined by the Department of Labor, Mine Safety and Health Administration. In order to be intrinsically, safe the power source combined with any additional electrical equipment connected to its output must be incapable of igniting or causing ignition of a mixture of flammable or combustible material in air in the mixture's most easily ignitable concentration.
The basic function of an intrinsically safe power source is to convert power from an available source to conditioned power. The conditioned power is regulated and filtered for specific use by other electrical equipment or devices. An intrinsically safe power source differs from a common power supply by the requirement that the conditioned power be intrinsically safe. The requirement of intrinsic safety imposes characteristics on the device that conflict with traditional performance requirements for a power supply.
FIG. 1 shows an idealized intrinsically safe power source 10. In power source 10, input power 12 is conditioned by power source 14 to produce conditioned power 16. Energy limiting means 18 acts on conditioned power 16 to produce intrinsically safe power 20. In the ideal case, the energy limiting means 18 does not have an adverse effect on the quality of power delivered to the load from power source 14. In reality, however, this is not possible. In particular, the regulation and transient response characteristics of the power source are severely restricted by energy limiting means 18. Accordingly, there is a need for an approach to significantly improve the level and quality of power delivered within the limits of intrinsic safety.
Intrinsic safety has been achieved on previous power supplies by two primary means. Both means are based on limiting the energy flow through the device. The first approach, shown in FIG. 2, is based on providing an adequate impedance or barrier to directly limit the energy delivery. The second approach, shown in FIG. 3, is based on providing a controlled series pass element or valve to stop the delivery of energy. Neither of these techniques addresses the energy stored in the output filter of the device or the energy stored in the load devices and cabling.
The impedance barrier concept is illustrated in FIG. 2. In this example, the barrier device is an energy limiting barrier resistor 22. The impedance barrier concept provides intrinsic safety by limiting the energy flow under all conditions. Unfortunately, the output power quality is a function of the load. As the load current increases, the voltage drop across the barrier resistor 22 increases. Accordingly, the voltage regulation characteristic at the intrinsically safe power terminals is poor. In addition, the total available power to the load is inherently limited by the barrier resistor.
U.S. Pat. No. 5,050,060 illustrates an intrinsically safe power supply unit that attempts to overcome the shortcomings of the barrier resistor approach. In the power supply unit of U.S. Pat. No. 5,050,060, a post regulator is applied after the barrier resistors to improve the power supply performance characteristics.
The controlled series pass element or valve approach is illustrated in FIG. 3. This approach relies on a detection method to determine whether or not the pass element should be turned off. Typically, this decision is based on monitoring the output of the power supply as illustrated by monitors 24. The control element is illustrated as a valve 26. Valve 26 could conceivably be any of several electronic devices including bipolar transistors, field effect transistors, and the like. The series pass element concept can be extended to include an array of more complicated implementations. These more complicated implementations combine the power source and energy limiting valve functions. In such implementations, the power source is turned off based on the output of the monitor circuit. The components providing the energy limiting and monitor functions are typically required to be triply redundant.
U.S. Pat. No. 4,455,509 illustrates a variation of the controlled pass element approach. In the intrinsically safe lighting system of U.S. Pat. No. 4,455,509, the detection of output anomalies is based on output current magnitude that is sensed by a resistive element. The reaction to an overcurrent condition diverts current from the gate firing circuitry for the output power circuit. This action turns off the delivery of power to the output. The intrinsically safe lighting system of U.S. Pat. No. 4,455,509 makes no attempt to remove trapped energy from the output power circuitry.